First of all: No, XP won't lag. Which means it will run at superb speed, according to all reports. There's no emulation involved, only a little tweaking on Bios/EFI level.

But you can go for your mac without doubts. I don't know if it's possible to banish OSX from you system (using boot camp).
But you should use OSX for a while. It's an AMAZING OS. Really. And I NEVER needed XP again. For anything. There might just be a few apps you'll need to run with XP, but for most things there's a good or better alternative.

Also be warned: The IR sensor on the mac and isight don't work under XP.

You shouldn't plan to buy a mac if you're going to use only windows. Mac Hardware still is made to work with mac software, and while windows will run, you might run into a few minor issues (but that's normal on any PC).

Boot camp really is meant to be a solution for people who HAVE to use XP once in while, not always. Macs are Mac OS. They'll stay Mac OS. They just happen to do windows, too.